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eMotorsports Cologne: Use Electric Strain Gauges

Company name, Country

Introduction

Since 2009, the students of eMotorsports Cologne and Cologne University of Applied Sciences have been building electric racing cars. Every season, the highly motivated team, with members from different courses, develops and produces racing cars for Formula Student competitions. Each vehicle is uniquely designed, driven by forward-looking innovation and commitment.

The 20-strong team wants to be a pioneer, promoting innovation and shaping the future of electromobility by creating a car that combines state-of-the-art engineering and cutting-edge technology.

Strain gauges from HBM are used to achieve this objective.

The current vehicle consists of a solid carbon monocoque with an aluminum honeycomb crash box. It is electrically operated (with 576 round cells protected by an aramid carbon housing) with four wheel-hub motors.

Further vehicle key data include:

  • 6.47 kWh battery capacity
  • 213 kg weight
  • 122 km/h maximum speed
  • 1055 Nm torque
  • 128 kW power
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Electric vehicles allow for a complete redesign owing to the new design and optimization options available. Energy efficiency through weight saving is an important factor here. With an increasing number of chassis components, conventional metal materials are replaced by fiber composite materials. New designs are possible due to the materials’ improved durability and fatigue properties.

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This enables a particularly high ratio of durability/weight to be achieved.  Moreover, any kind of structure can be created, for instance, suspension components can be optimally designed for external loads. However, these adjustments also change the overall vehicle dynamics. The vehicle behaves completely differently due to the influence of the center of gravity, a changed mass-damper system, and the modified forced excitations by the drive motor.

Electric foil strain gauges from HBM are used for chassis development and optimization. They help estimate the force effect.

The basic aim is to find an optimal design – minimum weight while ensuring full operational safety.

Therefore, the wishbone tubes are being optimized. Different diameters (17-22 mm) with different tube thicknesses (1-2 mm) and lengths are being investigated. The force flow into the monocoque varies depending on the tube length.

These days, development processes are largely characterized by data exchange between simulation and real-life tests. A key factor in this is that simulation and real-life tests mutually improve the model. The resulting model safely enables a broader, more comprehensive use of simulation, which in turn saves time and costs. This facilitates mapping of the load cases as realistically as possible in the simulation, based on the known real-life forces, thus improving future developments.

Using the example of a wishbone, the interaction of simulation and real-life tests (digital twin) is to be shown.

The wishbones are loaded in the chassis as pure tension-compression rods.

Real-life road test

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Simulation

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Wishbone Simulation

1 The first step is to use the tire data to determine traction and maximum tire grip in various driving conditions. Apart from the tires, the assembly space (rim, wheel carrier, etc.), including the dampers and the chassis, are taken into account.

2 A simulative "crash analysis" is also necessary to ensure that the motors do not touch the wishbone during vehicle movement (compression, rolling, etc.) and that the tie rod does not touch the rim during steering. Furthermore, the optimum ratio of wheel suspension travel to damper travel is sought. In this simulation, different load cases that affect the chassis are also simulated. For example:

  • Out-of-roundness
  • Curves, inside
  • Curves, outside
  • Braking
  • Acceleration
  • Braking

3 In the pilot test, the load cases result in specific force effects for each wishbone carbon tube (wishbone, tie rod):

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4 The CFRP tubes are dimensioned with a safety factor according to the forces calculated from the simulation. The tube length, the force acting on it, and, if necessary, a torque are entered at the far right. Different CFRP tubes can subsequently be compared depending on the outer and inner diameters to determine a design for use in the real-life test.

Wishbone Testing in a Real-Life Road Test

5 To verify that the simulated forces on the chassis components actually occur in the real-life components as well, testing of the real-life structure is required. The forces on the chassis components are determined during a road test. The use of HBM's electric foil strain gauges for force monitoring of relevant components is an important step in this process. For this purpose, the components with installed strain gauges are pre-adjusted using a reference force transducer so that they function as "force transducers".

A major advantage of the electrical strain gauges in this application is that they can be fully integrated into existing designs, making them perfect for mobile testing.  Basically, there are no issues caused by additional sensors that take up extra space and affect the real-life structure. Electric foil strain gauges are also very linear and achieve high accuracy when calibrated accurately.

6 The strain gauges are connected in a half-bridge configuration and installed on the wishbones. Pre-wired type K-CXY3 strain gauges are used for this purpose. These are attached to the carbon wishbone using the fast-curing Z70 adhesive. Using HBM's pre-wired strain gauges on composite materials is the ideal choice as there is no need for subsequent soldering on the material. Typical soldering temperatures can quickly damage such materials. The fluoropolymer cable on the strain gauge prevents it from sticking during gluing.

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The wishbones fitted with strain gauges are then adjusted on a universal tension-compression testing machine.

7 The wishbones used here are adjusted to up to 1 kN. Their ball-bearing support ensures an optimal clamping position (no tension of the components due to self-alignment). The bridge output voltage values (see further information on the Wheatstone bridge circuit LINK) are compared with the forces measured using the tension-compression testing machine.

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8 Every tension-compression bar is stored and parameterized as an individual sensor in the catman software The table format is selected for adjustment, and 10 measured values are stored.

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9 The wishbones fitted with strain gauges are then installed in the vehicle. The entire measuring chain consists of a 16-channel QuantumX MX1615B module, which is attached to the vehicle. The strain gauges are connected via cables using 4-wire technology. The measured data is stored on a CX22 data recorder. All measured data is synchronized via PTP2 for ease of evaluation. The strain gauges should be covered before their use in the field. ABM75 is ideal for this.

10 Finally, the real-life load data is recorded through the road test. This includes an evaluation of whether the components and the vehicle system offer adequate operational durability and how high the load is on each of the components.

Eventually, this data is used to improve component simulation:

Overview: Digital Twin Explained With the Example of the Wishbone

Real-life tests and simulations complement each other to enable the development of a perfect design.

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